Amebiasis is an important parasitic cause of morbidity and mortality in the developing world. It is thought that acquired immunity to infection with E. histolytica exists. Understanding the immune response to amebic invasion and determining the nature of the protective immune response are essential steps towards vaccine development The focus of this proposal is to determine whether individuals with evidence of past infection with pathogenic E. histolytica are protected from re-infection. This proposal builds on the long and productive collaborations of Drs. Mann and Braga. Work will be conducted in the community of Goncalves Dias, an urban slum six blocks from the laboratory site. In this community nearly half of the children have serologic evidence of amebic infection by age 10. The current studies are designed to test whether individuals previously infected with pathogenic E. histolytica (as determined by the presence of anti-galactose lectin and anti-amebic antibodies) are protected from re-infection. We will test sera and saliva from 900 individuals from Goncalves Dias for anti-galactose lectin and anti-amebic antibodies. We will also analyze stool samples from the same individuals for the presence of E. histolytica by microscopy, culture, and ELISA. We will assay both sera, saliva, and stool from the same individuals a year later and then determine if there is a correlation between presence of antibody and colonization rates. It is proposed that the successful completion of these studies will yield basic information about the immune response to E. histolytica and lead to new approaches to the prevention of amebiasis.